Your menstrual cycle begins on the first day of your period. Your cycle prepares your body for a possible pregnancy. The average cycle lasts between 24 and 38 days. When your period is approaching, you usually know it. Here’s a list of common physical and emotional signs that your period is on its way. Periods, also known as menstruation, occur when your uterus sheds its lining, causing blood and mucus to discharge from the vagina. 1 This usually occurs once a month if you are not pregnant and. Menstruation is the most visible phase of the menstrual cycle and its beginning is used as the marker between cycles. The first day of menstrual bleeding is the date used for the last menstrual period (LMP). What is a normal menstrual cycle? Your period — and the PMS symptoms that can accompany it — is just one part of your monthly menstrual cycle. Understand how to track your menstrual cycle and what to do about irregularities. Do you know when your last menstrual period began or how long it lasted? If not, it might be time to start paying attention. When you track your menstrual cycles, you learn what's typical for you. The menstrual phase is your actual period, or when you’re shedding your uterine lining and bleeding. It usually lasts 3 to 5 days, and often overlaps with the follicular phase. Discover 9 early signs your period is coming, from acne to cramps. Learn causes, PMS relief tips, and how to prepare for your monthly cycle. Menstruation (or period) happens monthly. As the uterus sheds its lining, vaginal bleeding follows. Menstrual blood is made of blood and tissue. Menstruation — aka having your period — is when blood and tissue from your uterus comes out of your vagina. It usually happens once every month.